At least once every six months, every gas pump in Missouri is tested by the Department of Weights and Measures to make sure it's accurately measuring a gallon. Search this database to see the results of your favorite station for tests from January 2005 until June of this year.
The "Pumps Tested" field tells you how many pumps were tested. Any pumps that are off by more than six cubic inches are dubbed "Out of Tolerance," and appear in the corresponding field.